Early environments that developing organisms encounter during ontogeny have profound and persistent effects throughout life. In human offspring, variation in early family environments and gestational endocrine environments can profoundly shape multiple biosocial parameters, including sociality and behavior, stress reactivity, pubertal timing, and later parental style. We will examine the impact of early environments on development in a marmoset model that is relevant to human development. Marmoset development occurs in a family system, with multiple caregivers providing qualitatively and quantitatively different forms of care. Further, mothers often conceive immediately postpartum, leading to elevated maternal steroid hormones (including androgens) that can impact both developing fetuses (via placental transfer) and nursing offspring (via milk-borne steroids), a condition often found in human females (e.g., polycystic ovarian syndrome). Finally, marmosets routinely give birth to some proportion of twins that, because of shared blood supply and exchange of stem cells between twins during gestation, are genetic chimeras. Marmoset parents exhibit different patterns of care toward chimeric infants than toward nonchimeric infants. The studies will address 3 aims: 1) Does variation in early care produce phenotypic variation in behavioral, physiological, and parental care?; 2) How is behavioral, somatic, and reproductive development shaped by pre- and postnatal exogenous steroids in marmosets?; and 3) What are the behavioral mechanisms by which adult caregivers differentiate between chimeric and nonchimeric offspring? Using a wellestablished colony of marmosets, we will evaluate the links between early care received by marmosets and later parental care toward their own offspring. Secondly, we will look for associations among maternal androgens during gestation and during lactation (including concentrations in milk) and subsequent biobehavioral parameters. Finally, we will carefully document qualitative patterns of offspring care toward infants that differ in chimeric status.